1. Field of the Invention
This invention relates to a testing device for thermal imagers and a method of testing thermal images.
2. Discussion of Prior Art
Thermal imagers test methods and apparatus are well known in the prior art. They generally require access to temperature controlled, substantially draught free environments in special purpose laboratories. They can be used to test imagers for minimum resolvable temperature difference (MRTD) or minimum detectable temperature difference (MDTD).
A typical prior art testing apparatus incorporates a set of pattern plates, usually five or more, for viewing by a thermal imager. Each plate has a respective pattern, and the patterns differ in spatial frequency. In use the plates are positioned one at a time in front of a heating/cooling element consisting of a high emissivity plate connected to a heating/cooling device, usually a peltier device. Such high emissivity plates are usually referred to as blackbodies, although strictly speaking their emissivities are slightly less than unity. The heating/cooling element is viewed through the pattern plate firstly when hot and secondly when cold relative to ambient temperature. The temperatures are adjusted manually.
Each plate is thus viewed twice and the test is time consuming. A curve of MRTD or MDTD versus spatial frequency is obtained, but the test does not provide a quick assessment of whether the thermal imager passes or fails. The most limiting factor however is the necessity to perform the test in a temperature controlled, substantially draught free laboratory environment. In addition the prior art method requires expensive apparatus, including the use of a radiometer to calibrate the system prior to use. There is no apparatus or method available in the prior art to enable realistic tests to be carried out in the field.